The present invention relates to an overcurrent detection circuit that can be built in a semiconductor integrated circuit, and in particular, to an overcurrent detection circuit that detects an overcurrent status of a current flowing through a power supply line to protect a power supply circuit when there is an overload on a power supply apparatus.
In common methods for detecting an overcurrent through a power supply line, a resistor is inserted into the power supply line to monitor a voltage drop caused by a current flowing through the resistor.
FIGS. 12, 13 and 14 show specific examples of conventional overcurrent detection circuits based on such methods.
FIG. 12 shows a general series regulator, in which a portion composed of a resistor 114 and a transistor 115 detects an overcurrent. This overcurrent detection section 113 is a constant current limiting type.
A threshold where a current I.sub.1 flowing through a power supply line 200 becomes excessive is referred to as I.sub.over. When the current I.sub.1 exceeds the threshold I.sub.over, a voltage drop caused by the resistor 114 increases above a base-emitter voltage V.sub.be of a transistor 115, and the transistor 115 is operated to reduce a collector-emitter voltage V.sub.ce. The base voltage of a power supply transistor 116 then decreases to reduce the variation of the current I.sub.1 flowing through the collector, thereby increasing the collector-emitter voltage V.sub.ce of the transistor 116. In other words, an output voltage decreases.
Therefore, the overcurrent detection section 113 has a function for limiting a current I.sub.1 flowing through the collector of the transistor 116 to protect the transistor 116 from damage caused by an overcurrent.
In FIG. 12, 111 is an input terminal; 112 is an output terminal; 117 and 118 are potential-dividing resistor; 119 is a reference voltage line; and 120 is an operational amplifier.
FIG. 13 shows a series regulator similar to that shown in FIG. 12 except for the configuration of the circuit of the overcurrent detection section. The overcurrent detection section 121 is composed of a transistor 115, a resistor 114, and resistors 122 and 123, and is a foldback current limiting type.
The overcurrent detection operation is almost the same as in the circuit in FIG. 12. A voltage drop caused by the resistor 114 activates the transistor 115, thereby reducing the base voltage of the power supply transistor 116. This operation reduces the output voltage, but the resistors 122 and 123 reduce the current I.sub.1 to protect the transistor 116 from damage caused by an overcurrent.
FIG. 14 is a circuit whose function is to detect an overcurrent only.
When a load is low, a voltage drop caused by the resistor 114 is small, so that a negative input terminal of a comparator 127 has a voltage higher than that of a positive input terminal connected to a connection between a diode 124 and a resistor 125. Therefore, the output from the comparator 127 is in a "Low" level.
When the load increases and the current I.sub.1 becomes the overcurrent I.sub.over to increase the voltage drop caused by the resistor 114 above the forward voltage V.sub.f of the diode 124, the voltage of the negative input terminal of the comparator 127 decreases to become less than the voltage of its positive input terminal. The output from the comparator reaches a "High" level, enabling an overcurrent to be detected.
The output from the comparator 120 is outputted from a detection terminal 126 as an overcurrent detection signal and is used to protect the power supply transistor on the input or output side.
In protecting a power supply transistor built in a regulator that is required to reduce the voltage between the output and input sides to 1 V or less, or a switching transistor inserted into a power supply line for switch-on and -off, the overcurrent can not be detected easily based on the voltage drop caused by the resistor as shown in FIGS. 12 to 14. Two possible causes are shown below.
First, in considering the voltage drop in a transistor, the voltage drop caused by a resistor connected in series to the transistor must be limited to 0.5 V or less in order to detect an overcurrent. Therefore, the base-emitter voltage V.sub.be of a bipolar transistor or the forward voltage V.sub.f of a diode can not be used as a reference voltage, and a reference voltage having a certain degree of accuracy can not be obtained.
Second, to reduce the voltage drop caused by the resistor, the resistance value is set to be 1 .OMEGA. or less when the output current becomes 100 mA or more. Therefore, the voltage drop caused by the resistor hardly varies in response to a small variation in an output current, thereby reducing the sensitivity for an overcurrent detection.
Due to these two points, the overcurrent detection using the voltage drop caused by the resistor generally has a low detection accuracy and is subject to variation or scattering. Furthermore, if an on-resistance of the power supply transistor or switching transistor is lower than the overcurrent detection resistance, this magnitude of the overcurrent detection resistance affects the voltage drop in the entire circuit including the transistor, thereby increasing losses relating to the voltage drop caused by the resistor.
Therefore, an object of the present invention is to provide an overcurrent detection circuit having a higher detection accuracy than the prior art.
Another object of the invention is to provide an overcurrent detection circuit that minimizes power losses.